Certain vehicles, such as light trucks and vans, frequently incorporate a dual rate suspension system, in which a low or soft spring rate is achieved during normal driving conditions and a higher spring rate is obtained under higher load conditions.
In the past, a dual spring rate has been achieved by utilizing multiple steel leaves of decreasing length. The main leaf will carry the load under normal driving conditions, but when an increased load is encountered the main leaf will deflect into contact with the secondary leaves to increase the spring rate to the designated level.
Recently fiber reinforced resin leaf springs have been utilized with vehicles in place of steel leaf springs and the use of the composite springs provides a distinct decrease in weight. U.S. Pat. No. 4,750,718 describes a dual rate leaf spring construction utilizing a composite spring formed of fiber reinforced resin. In accordance with the aforementioned patent, a secondary leaf spring is secured beneath and parallel to the main composite spring and the secondary spring has a substantially shorter length than the main spring and is therefore less flexible. Mounted on each end of the secondary spring is an elastomeric pad having an upwardly extending rounded head that is adapted to be engaged by the lower surface of the main leaf spring under load conditions. When a predetermined heavy load is applied to the vehicle, the main spring will be deflected downwardly into engagement with the pads to thereby provide an increased spring rate to accept the heavier loads.
There has been a recent trend toward lower profile vans and light trucks to provide more aerodynamic styling. The lower profile reduces the space for the suspension system, and as a result, conventional dual rate suspension systems, which utilize a plurality of leaf springs, cannot be used as multiple leaves would impinge on the ground clearance. Thus, there has been a need for a dual rate suspension system that can be incorporated with low profile vehicles having a limited space for the suspension system.